A. Field of the Invention
This invention generally relates to the art of providing an ornamental surface coating on an article of manufacture and, more particularly, the invention relates to preparing a metallic substrate for receiving a durable and ornamental surface coating.
B. Description of the Prior Art
Articles of manufacture having exposed ornamental metallic surfaces are well known in a variety of applications, such as jewelry or other personal accessories and furnishings, for example. In such applications, gold has been widely used as a surface coating because of its attractive appearance and corrosion resistance. Since precious metals, generally, are expensive to use as coatings for articles of manufacture, various techniques and substitute coatings have been developed which provide a less expensive but attractive external surface for such articles. For example, titanium nitride can be deposited on a metal substrate through the use of either physical or chemical vapor deposition techniques such that ornamental surface finishes may be achieved without the expense of using precious metals. Such a technique is disclosed in U.S. Pat. No. 4,252,862, issued to Nishida on Feb. 24, 1981.
A metallic surface finish which has recently become widely accepted for its pleasing, colorful appearance is a so-called "rainbow" finish. A rainbow finish may be achieved by a chemical anodization process wherein a preparation of a ten percent by weight ammonium sulfate solution in distilled water is placed in a suitable metallic container having means to heat the solution to between 120 and 140 degrees Fahrenheit and a variable DC voltage is applied with the tank serving essentially as the cathode and the part to be anodized serving essentially as the anode. The color patterns created in such a process is a function of the applied voltage and a working range of between 30 and 105 VDC is appropriate. As an alternative to the chemical anodization bath, brush plating may be used to provide surface color patterns by saturating the brush in the aforesaid ammonium sulfate solution and applying a variable DC voltage between the brush and substrate.
To produce attractive color anodized surface finishes, an article of manufacture may be made of solid, elemental titanium prepared by hydrofluoric acid etching prior to the electrochemical anodization process. The resulting process, however, has two limitations in the need for the hydrofluoric acid etch and in the need for an elemental titanium substrate. It is well known that hydrofluoric acid is highly corrosive, dangerous to use and is virtually restricted to use with cast or plate cut articles only. Accordingly, where it is desired that the articles be made of a drawn substrate or a substrate made from a metal other than titanium, the aforesaid color anodization process cannot be achieved.